1. Field of the Invention
The invention relates to specialty steel additive alloys containing calcium. More specifically, the invention is intended to provide a calcium-containing, carbon-free, silicon-free additive for steel which will give optimal effectiveness of the calcium added, without introducing deleterious materials into the steel and without the excessive turbulence accompanying other products and techniques.
2. Description of the Prior Art
Calcium is frequently added to steel to deoxidize, desulfurize, and to alter the characteristics of oxide and sulfide inclusions. The benefits of calcium in steel are well and amply documented in the technical literature. However, calcium boils at 1487.degree. C. (2709.degree. F.), whereas the molten steel to be treated is usually in the temperature range 1540.degree.-1650.degree. C. (2800.degree.-3000.degree. F.). Metallic calcium rapidly and often violently boils in the molten steel, resulting in poor efficiency, extensive re-oxidation of the steel, and inconsistent calcium effects.
To control the boiling of calcium during calcium addition, several techniques have been employed, both individually and in combination. In one technique, calcium is added at levels of about 0.25 kg/ton (0.025% by weight) as a relatively chemically stable compound with carbon (CaC.sub.2, "calcium carbide") or silicon (CaSi.sub.2, "calcium silicide"). The compound may be added by pneumatic injection through a refractory lance or by feeding a powder-core wire. While this method reduces boiling of calcium, each 0.01% calcium added to the steel introduces 0.01% carbon or 0.02% silicon. This level of addition is unacceptable for some grades of steel. Indeed, some grades of steel specify no deliberate addition of carbon or silicon.
Another common technique is to plunge masses of metallic calcium mechanically mixed or alloyed with large amounts of non-volatile materials such as steel, nickel, or manganese. The large amounts of non-volatile materials serve as a heat sink, slowing the rate at which the calcium boils. This technique results in excessive temperature losses and is often inconsistent in its calcium effects. Further, the non-volatile materials may themselves be deleterious in the steel being treated. While such plunging "alloys" or mixtures reduce the rate of calcium boiling, they do not prevent it. Consequently, deleterious effects of re-oxidation are not avoided.
Another common technique for calcium addition disclosed in U.S. Pat. Nos. 4,035,892; 4,671,820; and 4,698,095 to Ototani, et. al., utilizes metallic calcium or calcium alloys enclosed in a sheath of suitable metal, usually steel, which "wire" is then mechanically injected into the molten steel. In one case such a metallic calcium-cored wire is injected through a refractory lance. Obviously, those wires containing alloys of calcium with carbon or silicon suffer the disadvantages cited above for these alloys.
The injection of metallic calcium-cored wire is based upon the assumption that ferrostatic pressure will suffice to prevent boiling of the metallic calcium, if only the calcium can reach sufficient depths before reaching the temperature of the molten steel, and that boiling can be suppressed long enough for the liquid calcium to dissolve in the molten steel. However, this is unlikely to occur. As shown in Table I, the vapor pressure of calcium at steelmaking temperatures requires immersion depths (based on approximately 0.2 atmospheres per foot of depth) well in excess of the depths available in most steelmaking ladles.
TABLE I ______________________________________ T, .degree.F. VP.sub.Ca Atmos. VP.sub.Ca Gauge* Critical Depth. (Ft) ______________________________________ 2800 2.65 1.65 8.3 2850 3.13 2.13 10.6 2900 3.67 2.67 13.4 2950 4.35 3.35 16.8 3000 4.97 3.97 19.8 ______________________________________ *Note: Normal atmospheric pressure provides 1 atmosphere.
Moreover, injection using these techniques leads to globules of liquid calcium whose size approximates the diameter of the wire or plunging alloy used. Such wires typically are from 5 mm to 25 mm in diameter. The low density of the calcium (1.5 g/cc) compared to the density of the molten steel (approximately 7.15 g/cc) causes these globules to rise rapidly in the steel. Even if the ladle depth were such as to allow the calcium to be injected well below the Critical Depth, the globules quickly rise to the Critical Depth, where they flash to vapor.
As vapor, the calcium bubbles are very large, and very rapidly rise through the steel and the slag and are lost into the atmosphere. In passing through the steel and slag, the calcium bubbles induce strong stirring in the liquid steel, reducing the transit time of the calcium which follows.
The surface turbulence created by the calcium vapor increases the loss of heat from the steel. This must then be accommodated by increasing the temperature at the start of treatment. Increased starting temperatures, however, decrease the efficiency of the calcium addition, so more is needed and still more temperature is lost.
The rate of absorption of calcium into the molten steel is also severely limited by the very low solubility of calcium in steel, reported to be only 0.032% by weight in a pressurized system at 2925.degree. F. In most such systems, the rate limiting mechanism is diffusion through a quasi-static boundary layer whose thickness is usually in the range 0.1-1.0 mm. Industrial experience indicates that the combination of low transit time, auto-induced stirring, low solubility of calcium in steel and diffusion-limited mass transport results in low and erratic utilization of the added calcium and in highly variable calcium effects in the product steel.